



tioned above, the fluorescence is barely perceptible in the presence of the relatively large amount of visible light from the discharge in the lamp, and the direct ent of the nuorescence radiation is very diillcult and produces unsatisfactory results.

I have found, however. that the phosphorescence d the lead calcium silioatmpho phcr, after excitationby 1849 L radiation. was quite bright and lasted for an appreciable time so that quantitative measurements could bemade. The brightness of the phosphorescence varies with the intensity of the oficina! irradia tion of the phosphor by the 1849A. radiatioltsn that a measure of the phosphorescence provides. at the same time, a measure of the original radiation. The intensity of the phosphorescence, or after-glow, can be determined by-meansvof a1 suitable cell in conJunction with a phosphor scope. Such an instrument may take various forms; but. essentially. it comprises a surface. either a drum or a disc. coatedwith the phosphor and adapted to be continuously rotated'psst the exciting source and ,then into an area shielded from the exciting radiation and its inevitable acoomllmyins iight. the intensity oi the phosphorescence being measured in this shielded'area. The intensity of the phosphorescence may bedetermined either byutllizing a photocelland acmeasurements or. more simply.' by visual comparison withianl adjustable brightness area having the same color as the phosphorescence.

Referring to the single figure of the drawing, thereisshownadeviceforcarryingoutmy method which comprises a rotatable drumV I located within an enclosing box 2 which has been illustrated in dot-dashlin so the enclosed parts may be seen, it being understood that the boxis made ot opaque material and preferably painted with some radiation-absorbing substance. The sides oi' the drum are preferably intheform of circular discs of larger' diameter than the drum itself in order to prevent the escape of light past theedgesthereof, Thedrumismountedona shaft 3 which is adapted to. be rotated by a motor l. if desired. at a controllable speed. The cylindrical surface l of;r thev drum is coated with the-phosphor whichit is desired tok utilise. tor instance. lead-free calcium silicate manganeseactivated phosphor for measuring the 184.9I A radiationwhichhasbeendiscuasedhereim The source ot radiation t-is mcunted'in front of an aperture ora window I which. permits its radia-I tiontoilluminatearestrieteduea-ononesida ofthe drum. A phoincell or other lightqmealur ing device l is mountedon the opposite side of' the drum, preferably diametrically opposite the window l. Light bailes 9 and I. nt closely around the drum and prevent any light from penetratingtothefar sideofthebox wherein the light-measuring cell ismounted.

1n operation. the light source t. which may be a 4-watt mercury discharge ozone-producing lama-illuminates the drumthroughihe window 1. The drum is rotated and the excited phosphor is carried continuously past the baiiles into the region wherein the photocell l is mounted. Sipcethevisibie radiationfromthelamp leannot penetrate to the. Ian side of the box, the Dhotoceil responds only te the phosphore-scenes originaliyproducedbx the 1849 radiation. The cellthus provides .a true measure oi' the intensity of m9 radiation independently cf the other 4 long wave length radiations striking the drum on its near side opposite the window 1.

I have found that the speed of rotation of the drum is not critical and may be varied through wide limits without producing any appreciable change in the reading of the photocell. This appears to be due to the fact that, as the speed is increased. the decay in the phosphorescence isiclsi init. d thasame time. the duration-of excitation has been simultaneously decreased and the nal situation remains practically unchanged. F'or the lead-free calcium silicate phosphor. the phomhorescence appears to be caused to the extent oi approximately 92% by 1849 A radiation. 4% by 2537 A radiation, and the remainder. not exceeding 4%. by visible radiation. It will thus be seen that the device provides a widemeasureofsafetyasaresultof whicha small variation in the amount of visible radiation will produce practically no effect upon the reading of the photocell.

The measuring device. that is. the photocell l. may be. calibratedv to read the intensity directly. or comparative measurements may be made by stollllin the window 1 and determining the effective window size for obtaining a predetermined.

reading with the radiation whose intensity is to be. determined. The same results may be achieved' by varying the spacing between the lamp andthe aperture.

While I have described my new and improved method .in connection with a lead-free calcium silicate phosphor for measuring the intensity oi 1801A radiation. it will ofcourse be understood that the method' is equally applicable to use with other phosphore and to the measurement of other radiations. For instance, an antimony-free calcium halophosphatc phosphor activated with manganese (and wherein the halogen is chlorine or iluorine or both) may be used for measuring 1849A radiation. Likewise, other phosphore may be used. with suitable variations in technique. for measuring other radiations, such as. for instance. zinc silicate activated with manganese. or calcium halophosphate (activated with antimony and manganese) tor measuring 2537 A radiation.

Likewise. by employing suitable nlters. the phosphor designated as DR. and composed of calcium phosphate activated with both cerous cerium and manganese. may be used to measure erythcmal (sun-tanning) radiation (about 2800- 3200`A) The appended claims are therefore intended to cover any such variations coming withinl thejtrue spirit and scope of the invention.

WhatI. claim as new and desire to secure by Letters Patent of the United States is:

1. The method of measuring the intensity of radiation oi' a particular wave length from a source in the presence of undesired radiations of other wave lengths. which comprises energizing the source at a constant level, moving a surface covered with a phosphor responsive in phosphorescenee to said particular wave length and covely. unrponsive therein to said other wavelengths past the. source. and into an area shielded from said radiatiom. and measuring the intensity of radiation from said phosphor in said shielded area.

2. The method ofl determining the intensity of a particular radiation from a lamp in the pressence of other radiations of different wave lengths, which comprises energizing the lamp at a constant level, rotating a surface covered with a phosphorescent material responsive substantially exclusively to said particular radiation 

